Apple tree rootstock named ‘G.41’

ABSTRACT

A new and distinct variety of apple tree rootstock  Malus domestica×Malus robusta  hybrid ‘G.41’ is described herein. The new variety is a dwarfing rootstock that is resistant to fire blight ( Erwinia amylovora ) and crown rot ( Phytophthora cactorum ). The ‘G.41’ rootstock is useful in that it can be propagated clonally and used as a rootstock or root system for apple trees as well as for interstems of apple trees.

Genus and species: Malus domestica×Malus robusta hybrid.

Variety denomination: ‘G.41’.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention described here is a new variety of apple tree rootstock,Malus domestica×Malus robusta hybrid, hereinafter referred to as ‘G.41’.‘G.41’ originated from a planned cross in 1975 in Geneva, N.Y. The newvariety is a dwarfing rootstock that is resistant to fire blight andcrown rot. It is precocious and highly productive, and can be used as arootstock as well as for interstems of apple trees.

In the spring of 1975, pollen from a Malus robusta ‘Robusta 5’ appletree was applied to emasculated flowers of a Malus domestica ‘Malling27’ apple tree at the New York State Agricultural Experiment Station(“NYSAES”), Cornell University, Geneva, N.Y. In the fall of 1975,approximately 500 seeds resulting from this pollination were extractedfrom mature fruit derived from this cross. In the winter of 1975-76, theseeds were stratified and planted in large flats under conditionseffective to germinate seeds and obtain seedlings. When germinatedseedlings were about 2.5 cm tall they were inoculated with a mixture ofisolates of the fungus Phytophthora cactorum (the causal agent of crownand root rots). The flats were flooded to mid-hypocotyl level and keptat 23° C. for one week. Surviving seedlings were transplanted intoindividual pots.

In the summer of 1976, each of the transplanted seedlings was inoculatedwith approximately 10⁶ colony forming units of the Ea 273 strain of thefire blight bacterium Erwinia amylovora by inserting a 26-gaugehypodermic syringe needle into the shoot tip. The seedling designated at#041 was one of the survivors of this battery of inoculations from thesame cross. All the surviving plants were transplanted to the Loomisfield (NYSAES, Cornell University, Geneva, N.Y.) in the fall of 1976 andallowed to grow side shoots for propagation/evaluation. In 1978, ‘G.41’was evaluated for rooting ability, lack of spine production, and lowroot brittleness in a layering bed (stool bed). In 1982, 4 finishedtrees with ‘G.041’ rootstock were planted in a first test orchard on theLoomis farm (NYSAES, Cornell University, Geneva, N.Y.) with Malusdomestica cv. ‘Northern Spy’ grafted onto this rootstock as the scioncultivar. In 1983, two trees of the ‘G.41’ rootstock were also plantedwith Malus domestica cv. ‘Golden Delicious’ as the scion cultivar. Thisrootstock performed well (top 20% of many rootstocks tested) in thesefirst test trials and more material was propagated by stool bed to beentered into new trials at the Research South Farm (NYSAES, CornellUniversity, Geneva, N.Y.). The ‘G.41’ rootstock performed well with allthe scion cultivars that were tested. In summer 2002, 40 rootstockliners were inoculated with four different strains of fire blight (tenliners per strain) in the Plant Genetic Resources Unit greenhouse(NYSAES, Cornell University, Geneva, N.Y.) of the U.S. Department ofAgriculture Agricultural Research Service. As a result, the applerootstock ‘G.41’ was classified as resistant to the most virulentstrains of fire blight.

Asexual reproduction of the ‘G.41’ apple rootstock has been achievedusing the traditional method of clonally propagating apple rootstocks.In particular, the original seedling of the ‘G.41’ apple rootstock wasplanted in the Loomis field (NYSAES, Cornell University, Geneva, N.Y.)and allowed to develop into a “mother plant.” The ‘G.41’ mother plantwas then used to obtain rooted liners using conventional layeringprocedures. The resulting liners were then planted in a row to generatea layering stool bed (also referred to as the “mother stool bed”). Theliving tissues (i.e. leaves, stems, roots, buds, and spines) of themother stool bed were observed to be identical to secondary and tertiarystool bed plants. In addition to conventional layering, the ‘G.41’ applerootstock variety has been asexually reproduced by root cuttings, bybudding and grafting onto seedling and clonal rootstocks, and by tissueculture. Observations of trees from these propagations indicate that alltrees have proven true to type and identical in all appearances to theoriginal tree.

COMPARISON WITH PARENTAL AND KNOWN CULTIVARS

The seed parent Malus domestica ‘Malling 27’ is a super-dwarfingrootstock, i.e., trees grown on this rootstock are 15 to 20 percent thesize of a standard self-rooted seedling tree. ‘Malling 27’ is known toinduce good precocity to the scion (i.e., the ability to induce earlyreproductive development in the scion) and has high yield efficiency.‘Malling 27’ may induce small fruit size in some scion cultivars.‘Malling 27’ plants produce no spines, are poorly anchored, are verysensitive to winter cold, and drought, and are susceptible to the woollyapple aphid (Eriosoma lanigerum) and to fire blight (Erwinia amylovora).

The pollen parent Malus robusta ‘Robusta 5’ is a non-dwarfing rootstock,i.e. trees on this rootstock are the same size as a standard self-rootedtree. ‘Robusta 5’ does not induce precocity to the scion and is nothighly yield efficient. Juvenile plants of ‘Robusta 5’ produce manyspines. ‘Robusta 5’ breaks buds very early in the spring and is winterhardy. It is resistant to powdery mildew (Podosphaera leucotricha) andfire blight, and is immune to the woolly apple aphid. Further, ‘Robusta5’ has shown tolerance to the specific replant disease complex.

The ‘G.41’ apple rootstock of the present invention has a combination ofqualities that distinguishes it from its parental plants (i.e., ‘Malling27’ and ‘Robusta 5’). For example, although ‘G.41’ has dwarfingproperties derived from ‘Malling 27’, it is different from its dwarfingparent (i.e., ‘Malling 27’) because it produces a tree that is 30 to 35percent of the size of a standard self-rooted seedling tree. Thus ‘G.41’belongs to a different dwarfing vigor class than its ‘Malling 27’parent. In particular, ‘G.41’ has dwarfing characteristics that aresimilar to the industry standard of Malus domestica ‘Malling 9’. While‘G.41’ is like ‘Malling 27’ in that they both induce precocity to thescion and are highly yield efficient, unlike ‘Malling 27’, ‘G.41’ doesnot induce small fruit size.

With regard to its ‘Robusta 5’ parent, ‘G.41’ is distinguishable in thatit is a dwarfing rootstock and is highly yield efficient, while ‘Robusta5’ is not. However, like ‘Robusta 5’, ‘G.41’ is resistant to fire blightand powdery mildew. In addition, ‘G.41’ has been shown to have sometolerance to the replant disease complex.

As discussed above, ‘G.41’ is most similar to ‘Malling 9’ in terms ofdwarfing class and yield efficiency. However, ‘G.41’ is resistant tofire blight whereas ‘Malling 9’ and other rootstocks in its market classare not.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a new and distinct variety of appletree rootstock named ‘G.41.’ The ‘G.41’ apple tree rootstock is adwarfing rootstock that is resistant to fire blight (Erwinia amylovora)and crown rot (Phytophthora cactorum). The ‘G.41’ rootstock is useful inthat it can be propagated clonally and used as a rootstock or rootsystem for apple trees as well as for interstems of apple trees. While‘G.41’ rootstock is a hybrid from a cross between ‘Malling 27’ and‘Robusta 5’, it is distinct from its parent cultivars in terms ofdwarfing. The apple tree rootstock ‘G.41’ is less dwarfing than thesuper-dwarfing ‘Malling 27’ and more dwarfing than the non-dwarfingparent ‘Robusta 5’. Unlike ‘Malling 27’, ‘G.41’ is resistant to fireblight. Although ‘G.41’ is in the same dwarfing market class as ‘Malling9’, it is distinguishable from ‘Malling 9’ because ‘G.41’ is resistantto fire blight and precocious whereas ‘Malling 9’ is extremelysusceptible.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a dormant shoot (including buds and bark).

FIG. 2 shows the adaxial (upper) lamina surface of a mature leaf.

FIG. 3 shows flowers.

FIG. 4 shows the mature fruit.

FIG. 5 shows a nursery liner with a dormant apex.

DETAILED BOTANICAL DESCRIPTION OF THE INVENTION

The following description of apple tree rootstock ‘G.41’ containsreferences to color names taken from The Royal Horticultural SocietyColour Chart (R.H.S.), 2001 edition. Botanical descriptions follow theManual of Cultivated Plants (Bailey, 1949).

-   Tree:    -   -   Habit.—The unbudded tree of ‘G.41’ is a small shrub            typically standing about 2 meters tall by about 2 meters            wide when 6 years old. There is no single dominant trunk.            Instead there are several shoots arising from the roots. The            growth rate of the tree of the ‘G.41’ rootstock is            moderately slow, with early cessation of growth typically            occurring about two or three weeks prior to ‘Malling 9’ (an            apple rootstock having the same dwarfing class). Spring bud            break is mid season to moderately late. Very few suckers            (i.e., new shoots emerging from below ground) are produced.            Liners planted in the nursery stop apical growth mid season.            The apical bud in these plants is pubescent Greyed-Green            (191D).        -   Productivity.—In an intermediate trial performed at the            NYSAES (Cornell University, Geneva, N.Y.), the ‘G.41’            rootstock received the cultivar ‘Empire’ (Malus domestica)            as the scion and was compared to the Malus domestica check            rootstocks M.9 EMLA, M.26 and M.7. ‘G.41’ was shown to have            statistically higher (p≦0.05) yield efficiency (kg yield/cm²            trunk cross sectional area) than all the check rootstocks.        -   Precocity.—Scion cultivars budded on ‘G.41’ exhibit the same            precocity as those budded on M.9.        -   Fertility (Fecundity).—The ‘G.41’ plant produces flowers and            fruits regularly.-   Dormant shoots (buds and bark):    -   -   Dormant mature shoots.—Color: Greyed-Orange (RHS 175B) where            exposed to full sunlight grading to Greyed-Orange (RHS 170B)            with diminished light exposure. Texture: Very light            pubescence which gradually disappears in older tissues.            Size: 30-60 cm long; may have some spines. Spines            (one-year-old shoots): Length: 1-4 cm. Color: Greyed-Red            (RHS181A).        -   Axillary buds.—Size: Length: 5-8 mm. Width: 3 mm. Shape:            Obtuse, sessile, somewhat appressed and flattened. Texture:            Some pubescence.        -   Apical buds.—Size: Length: 8-10 mm. Texture: Pubescent.        -   Bark on three-year-old shoots.—Color: Greyed-Brown (RHS            199C). Texture: Longitudinal cracks in the bark are filled            with a Greyed-Orange (RHS 163B) suberine tissue. Lenticels:            Color: Greyed-Orange (RHS 163B). Size: 0.3-0.4 mm in            diameter. Quantity: 1-2 lenticels per cm².-   Leaves:    -   -   Mature leaves.—Leaf arrangement: Alternate. Shape: Simple,            recurved, oblong-ovate. Size: Length: 100 mm. Width: 50-60            mm at the widest point. Laminae: Somewhat wavy. Apex:            Acuminate. Base: Nearly symmetrical rounded. Margin: Acutely            serrated, with about 5 serrations per cm. Upper surface:            Color: Green (RHS 126B). Texture: Glabrous and translucent.            Lower surface: Color: Green (RHS 146C). Texture: Somewhat            pubescent. Venation: Netted. Leaf poise: 15°-25° from the            shoot, depending on shoot orientation. Stipules: Length:            8 mm. Width: 2 mm. Petioles: Diameter: 2 mm. Color:            Gradation of Green (RHS 140A) to Greyed-Red (RHS 179A)            depending on low or high exposure to light.-   Flowers:    -   -   Habit.—Flowers borne on spurs, shoot terminals, and from            lateral buds on growth from previous season.        -   Flower diameter.—40 mm.        -   Fragrance.—None.        -   Buds.—Location: Located on spurs and terminals; are mixed,            typically producing a truss of 5 to 6 flowers and one bourse            shoot. Shape: Obtuse, sessile and somewhat appressed. Size:            Length: 4 mm. Width: 3 mm. Habit: Buds near the base of the            shoot of the previous season usually produce 3 to 5 flowers            and a single short shoot; mid-shoot buds may have 2-4            flowers; and more distal buds are usually vegetative.        -   Petals.—Size: Length: 25 mm. Width: 18 mm. Shape: Spatulate.            Apex: Obtuse. Margin: Smooth. Texture: Smooth. Color: Closed            petals: Red (RHS 54A). Open petals: White (RHS 155D) with            Red (RHS 54A) striations in some of the petals.        -   Reproductive organs.—Pistils: Number: Length: 6 mm. Color:            Yellow-white (RHS 145B). Stamens: Number: Length: 4 mm.            Color: White (RHS 155D). Anther color: Yellow-Brown (RHS            167D).-   Fruit:    -   -   Mature fruit.—Size: Height: 30-35 mm. Diameter: 30-35 mm.            Shape: Obloid. Color: Partial Yellow-Orange skin (RHS 21A)            with Red (RHS 45C) blush overtones depending on the exposure            to the sun. Sepals: Persisting on a protruding calyx. Flesh:            Taste: Astringent (not meant for consumption). The mean size            of calyx under the fruit: 9.2 mm. Aperture eye: closed.            Color: Yellow-Orange (RHS 17C) with some Orange (RHS 32A)            around the seed cavity. Seed: Color: Greyed-Red (RHS 179A),            translucent. Shape: Tear drop shape. Size: Length: 50-60 mm.            Diameter: 25-35 mm at the widest point. Number: Generally            five seed per fruit.

Disease resistance: As described above, the ‘G.41’ rootstock of thepresent invention exhibits resistance to fire blight. The percent lesionmeasured after inoculation of potted liners in the greenhouse using fourdifferent strains of E. amylovora was negligible for two of the strainsand moderate for the other two, indicating a specific resistance to thebacterium. The ‘G.41’ rootstock, having survived the inoculation withcrown and root rot, is also considered resistant to crown and root rotscaused by Phytophthora cactorum.

1. A new and distinct variety of apple tree named ‘G.41’ herein described and illustrated. 